1. Field of the lnvention
This invention relates to cooling integrated circuit chip packages and more particularly it relates to improved convective cooling of high power devices in molded plastic packages.
2. Prior Art
Various heat sinking configurations for the enhancement of integrated circuit chip packages are known. In many instances, additional spring clamps are used to assist in the assembly of the heat sink to the circuit package. Clamps require additional parts with their associated costs and increased handling and inventory overhead. A spring/clamp attach technique also fails to reduce potential thermal interface problems which may occur when two non-flat surfaces are mated together. This interface mismatch increases the thermal resistance to heat flow and, thus, reduces the amount of power that can be dissipated by the assembled heat sink.
Available techniques to reduce the above noted interface thermal resistance problem include the utilization of a material such as a thermal grease or a thermal epoxy between the circuit package and the thermal enhancement (heat sink).
Results obtained from characterizing bonded heat sink components indicate that a heat sink sufficiently enhances the power handling capability of the circuit package. FIG. 1 shows, for a three-fin heat sink, an empirically developed plot of temperature rise of a plastic packaged circuit chip versus power dissipation of that circuit chip. Temperature rise from the upper surface of a plastic package (case) to the module ambient (air) is plotted on the vertical axis as a function of the power dissipation of the circuit chip on the horizontal axis. Comparison of the two plots indicates increased power handling capability for the same temperature differential when the plastic package is enhanced with a three-fin heat sink. This increase in power dissipation relates directly to an increase in performance, function, and reliability.
FIG. 2 exemplifies conventional surface mount plastic package heat sinks. A leaded component (not visible) is packaged in molded plastic housing 4, also known as a flat pack. Component leads 6 extend from housing 4, which is typically formed by joining two separately molded members 8 and 10 at the interface indicated by dotted line 12.
Interface material 14, typically thermal glue or epoxy, is sized to conform to the upper planar surface 16 of housing 4. An extruded finned heat sink 18 is retained in heat conducting relation to component housing 4 by interface material 14.
One difficulty with such a thermal enhancement technique is the attendant requirement for special tooling to accurately align and orient the heat sink and interface material with respect to the component during epoxy curing operation(s) in order to avoid any misalignment which would have a negative impact on efficient utilization of the thermal enhancement.
Further, epoxy is difficult to handle in a rework environment since it forms a strong bond the breaking of which increases the chances of damage to the assembly.
Some of these problems may be solved with the use of a non-curing thermal grease as the interface material. However, a thermal grease still must be applied to one of the two mating surfaces, and its use induces alignment problems since it does not "set". Although thermal grease is very reworkable, it is not stable. This feature implies that a slip could occur between the two bonded surfaces which would reduce the effectiveness of the enhancement. Thermal grease could leak out if not adequately confined; and its material properties may change with time creating a material interface not representative of initial design requirements.
Prior art heat dissipation methods are adequate so long as circuit package design and spacing constraints are such that an additional spring assembly can be incorporated into the product design. When the assembly procedure is such that a curing cycle does not add unacceptable expense in the form of additional ovens, floor space, utility consumption, material handling, etc., the use of epoxy bond is acceptable. If material stability is not a problem, then thermal grease interface techniques may be used.
There exists, therefore, a need for an efficient, cost effective, reworkable thermal enhancement arrangement for enabling use of high power devices in low end systems.
IBM Technical Disclosure Bulletin, Vol. 22, No. 3, 8/79. p. 960 to DeMaine et al describes a field removable, replaceable, and reusable heat sink. Attachment of the heat sink to the module is accomplished by means of a heat or chemically shrinkable plastic collar. IBM Technical Disclosure Bulletin, Vol. 28, No. 12, 5/86. p. 5172 to Lee et al discloses a heat sink adapted for attachment to a module with a spring clip.
IBM Technical Disclosure Bulletin, Vol. 28, No. 12, 5/86. p. 5531 to Curtis et al discloses a technique for thermally enhancing plastic packaged modules by bonding ceramic to the device lead frame prior to encapsulation.
Presently, it is desirable to use automated assembly procedures with reduced assembly operations. Included in such procedures is an additional desire to stock minimum parts, and to have a reworkable enhancement assembly technique which performs uniformly with respect to the lifetime of the circuit package. In this case prior art circuit package thermal enhancement attachment techniques are inadequate.
Accordingly, it is an object of this invention to provide an improved, simplified attachment technique for the assembly of a reworkable convective heat sink to a circuit package.
It is a further object of the present invention to attach the convective heat sink to a circuit package without introducing new materials and/or processes to the assembly operation.
It is a still further object of this invention to provide an assembly technique that is not restricted to a single convective heat sink design.
It is still another object of this invention to insure that the convective heat sink is correctly oriented with respect to the orientation of the circuit package and the cooling flow.